they do write most of their own songs but they have some writers that they love to work with like kamille, jess glynn and ed Sheeran ... mnek produces a lot of their songs too
To answer your question, it is a mix of both. Some songwriters, such as Kamille and Jess Glynne, work with them a lot, like their songs More Than Words(which Kamille sang on) and Woman Like Me(which Ed Sheeran also wrote on.) Other famous people write with them to, like Normani, who wrote their single Bounce Back. They also write many of their own songs, and Jade and Leigh-Anne actually became official songwriters last year. Perrie and Jesy also write many of their songs, and they are very involved in the producing process. I would recommend watching "An Introduction to Little Mix" by Nikola Mixer, as it gives a very nice insight to their lives in and out of the industry. Some of the songs they have written on include Woman's World from their LM5 album, Wings from their DNA album, and many many others.
Actually it's more than a mix, usually the girls assist in writing 80% of their songs on their albums, but Kamille (basically the 5th LM member) writes a HUGE majority of their songs...and has written almost all of their hits...but this song you reacted to, Leigh-Anne, Perrie and Jade co-wrote with Kamille and 2 other notable writers, but Kamille co-produced this track as well....she's just an AMAZING song writer to put it mildly!
This song isn't from that album, not sure if you knew that or not, but this is from their upcoming album which will be their 6th, but "Wasabi" and "Strip" are from the previous album "LM5"
Little mix have recorded so many fine songs. 'Lightening' off the Get Weird album is most excellent, different, but excellent. Also, 'Little me' 'Wasabi' and 'The cure' You could also have a listen to their live lounge cover of Love on the brain. Really enjoying your reactions.
If you react to any LM album....it should definitely be "Glory Days: Platinum Edition" it's probably their best work to date,.....it just shows what these girls are Truly about! and it was their first and only #1 album in the UK.